In recent years, as a power generation system which is mild to the global environment and clean, fuel cells have drawn attention. Fuel cells are classified, depending on kinds of electrolytes, into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, etc. Among them, polymer electrolyte fuel cells are tried to be applied as power sources for electric vehicles, power sources for portable apparatuses, and, further, applied to domestic cogeneration systems utilizing electricity and heat at the same time, from the viewpoints of workability at low temperatures, miniaturization and lightening, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of a polymer electrolyte membrane having ion conductivity (the ion is usually proton), electrode catalyst layers comprising a platinum group metal catalyst supported on carbon powder and an ion-conductive binder comprising a polymer electrolyte are formed, respectively. On the outsides of the electrode catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the electrode catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are bonded to the electrolyte membrane so that the electrode catalyst layers can face to the electrolyte membrane, respectively, is called a membrane-electrode assembly (MEA). On both sides of the membrane-electrode assembly, separators having electric conductivity and gastightness are placed. Gas paths supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane-electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
Further, in addition to such proton exchange-type fuel cells, anion exchange-type fuel cells using an anion-conductive membrane and an anion-conductive binder (the anions are usually hydroxide ions) are also studied. The constitution of a polymer electrolyte fuel cell in this case is basically the same as in the proton exchange-type fuel cell except that an anion-conductive membrane and an anion-conductive binder are used in place of the proton-conductive membrane and the proton-conductive binder, respectively. As to the mechanism of generation of electric energy, oxygen, water and electrons react at the oxygen electrode to form hydroxide ions, the hydroxide ions pass through the anion-conductive membrane and react with hydrogen at the fuel electrode to form water and electrons, and the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode and react again with oxygen and water to form hydroxide ions. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
The above electrode reactions take place at three-phase interfaces formed by a gaseous phase as a supplying path of the fuel gas or oxidant gas, a liquid phase as an ion path and a solid phase as an electron path. The ion-conductive binder is used for the purpose of binding the catalyst and heightening the utilization efficiency of the catalyst by mediating the transfer of protons or hydroxide ions from the electrode catalyst layer to the electrolyte membrane. Therefore, catalyst particles not contacting with the ion path formed by the ion-conductive binder cannot take part in the formation of the three-phase interfaces, and it is hard for such particles to contribute to the reaction. Further, in order to obtain high efficiency, the minute structural design of the electrode catalyst layer including pore structure for diffusing fuel gas or oxidant gas, the dispersion state of the catalyst, etc. becomes important. Further, at the gas diffusion electrode parts, there arises a case wherein the catalyst surface is covered with water contained in the reaction gases or water formed at the oxygen electrode or the fuel electrode, and the fuel gas or the oxidant gas cannot contact with the catalyst surface, and as a result, power generation is stopped, or a case wherein such water prevents the fuel gas or oxidant gas from being supplied or discharged to stop the electrode reaction. Therefore, the water repellency of the gas diffusion electrode part is required.
As a method for preparation of a membrane-electrode assembly, a method is known which comprises arranging a gas diffusion electrode prepared by applying a catalyst slurry wherein an electrode catalyst, a polymer electrolyte, and so on are dispersed by mixing in a solvent onto a gas diffusion base material, and drying it, and a polymer electrolyte membrane so as to be the order of the gas diffusion electrode/the polymer electrolyte membrane/the gas diffusion electrode, and bonding the resulting composite by a hot press or the like.
As a polymer electrolyte membrane, Nafion (registered trademark of Dupont Co., which is the same hereinafter) which is a perfluorocarbonsulfonic acid polymer, is generally used from the reason that it is chemically stable. A Nafion membrane has such a structure that spherical clusters having a size of the order of several nm are mutually connected via channels having a narrow interval of the order of 1 nm, by action of strong hydrophobicity of the main chains and hydrophilicity of the sulfonic acid groups, and shows high ion conductivity. Nafion is also used in an electrode catalyst layer in order to form three-phase interfaces acting as electrode reaction sites.
In usual membrane-electrode assemblies, Nafion is used both as a polymer electrolyte membrane and an electrolyte in the electrode catalyst layer. Namely, since electrolytes of the same composition are used, it is comparatively easy to obtain good bonding strength and a good electric bonding state. However, even when Nafion is used both as a polymer electrolyte membrane and an electrolyte in the electrode catalyst layer, there is interface resistance between the membrane and the electrode, and it is pointed out that internal loss of the cell caused by interface resistance arises to lower power generation efficiency. Since the electrode catalyst layer has a porous structure, the surface of the electrode catalyst layer has an uneven structure, and a problem is also pointed out that the reaction area of the electrode catalyst layer is decreased because the electrolyte membrane does not follow the uneven structure. Especially, when an electrolyte other than Nafion such as a hydrocarbon electrolyte is used as either of the electrolyte membrane and the electrolyte in the electrode catalyst layer, the problem of poor bonding between the membrane and the electrode caused by different kind of materials arises strikingly. From the viewpoint of securing a long-term reliability of fuel cells and, further, from the viewpoint of enhancing power generation efficiency, a method for forming good membrane-electrode bonding interfaces is important, and development of a membrane-electrode assembly having low interface resistance between the membrane and the electrode is desired.
As a method for improving bonding properties between the electrolyte membrane and the electrodes, it is proposed, for example, to put an ion conductor intermediate layer having proton conductivity between the layers of the electrolyte membrane and the electrode(s) (Patent Document 1). By using the ion conductor intermediate layer being softer than the electrolyte membrane and the electrode(s), the ion conductor is dug into the uneven electrode surface to enhance bonding properties. As another example, a method to make interface resistance smaller by making electron-conductive particles present between the layers of the electrolyte membrane and the electrode(s) is proposed (Patent Document 2). It is disclosed to enlarge the surface area of bonding interfaces and reduce interface resistance by forming an uneven structure at the interface part of the electrolyte membrane and the electrode(s). As a still another example, a method for improving bonding properties between the electrolyte membrane and the electrode(s) by making an intermediate layer comprising the same electrolyte as the electrolyte membrane and a carbon material composing an electrode catalyst layer (electron-conductive particles) present between the layers of an electrolyte membrane and an electrode, and preventing the membrane from damage and, at the same time, relieving stress applied to the membrane is proposed (Patent Document 3).    Patent Document 1: JP 2000-195527 A    Patent Document 2: JP 8-64221 A    Patent Document 3: JP 2005-190749 A